Sound recording and reproducing system



Dec. 13, 1938. s. "SEELEY 2,139,916

SOUND RECORDING AND REPRODUCING SYSTEM Original Filed Jan. 19, 1931 Z-Sheets-Sheet 1 (I I u 0 +20 Trezuency in cycles ps1 ssc. IHVEHTUR STUART W SEELEY wITNE s s Q BY Dec. 13, 1938. s. w. SEELEY 2,139,916

SOUND RECORDING AND RE PRODUCING SYSTEM I Original Filed Jan. 19, 1931 2 Sheets-Sheet 2 level f'regueney 1.1: cycigs per sec.

\D eci. bets from 50 cycle revel i. 1.

Freguency in cycles per sec.

INYENTDR ETUARTWT SEELEY Wrmzss v BY w w %m {M I fh'ruRnEY Patented Dec. 13, 1938 UNITED STATES PATENT OFFEQE SOUND RECORDING AND REPRODUCING SYSTEM Stuart W. Seeley, Jackson, Mich., assignor, by direct and mesne assignments, to Radio Corporation of America, New York, N. Y.

Original application January 19, 1931, Serial No. 509,693. Divided and this application April 15, 1935, Serial No. 16,296

2 Claims. (01. 179-1004) This invention relates to improvements in the audio frequency spectrum and, therefore, if sound recording and reproducing systems, and is 500 cycles represents the half-way mark on the a division of my copending application Serial No. tonal scale, substantially 90% of the disturbing 509,693 filed January 19, 1931. frequencies will be present in the upper half In the present day art of recording sound, of the audio frequency spectrum. From a study particularly on disc records, it is well known that of the physiological characteristics of the ear, it the cutting mechanism used in making such rec is known that interference in the upper half of ords must, of necessity, be so governed that the the audio spectrum is more objectionable than lateral velocity of the cutting tool is not absointerference in the lower half of the spectrum. l lutely proportional to the applied voltage at all This is because of the well known fact that the ii frequencies. This is because of the fact that vthreshold value of audibility of the human ear the lower frequencies tend to overlap from one is far higher at the lower frequencies than at the groove to the other and thus cause the reprofrequencies ranging form 1,000 to 5,000 cycles. ducing needle to jump from one groove to the It is, therefore, desirable to eliminate particularly next. An examination of records designed to the interference present in the upper half of i give constant output at various frequencies in the audio spectrum. the audio band, discloses the fact that the am- I have found that the difficulties due to nonplitude of the wave form of the track is approxiuniformity of response and to surface noise can mately inversely proportional to the frequency be substantially overcome by raising the energy and thus the lowest notes to be reproduced must level, during the recording process, of the higher 20 be attenuated before entering the cutting tool frequencies of the audio spectrum to a point well in order that the amplitude of the wave form of above the energy level of the surface noise in the path would not be too great. that region. This may be accomplished by using It is also well known in the. art that the re an amplifier ahead of the record cutting tool production of phonograph records is invariably having a rising gain characteristic with the fre- 25 accompanied by a certain amount of surface quency. Then, in order to facilitate a flat frenoise or what is commonly known as record quency response in the reproducing system,Iproscratch. If this is allowed to remain in the repose to utilize an amplifier following the record produced sound, it is exceedingly objectionable pick-up in which the gain is approximately inand if it is removed by means of electrical or versely proportional to the frequency. Since the 30 mechanical filters, certain desirable portions of lower frequencies of the sound to be recorded the recorded frequencies are lost. normally impart a greater amplitude of wave I have found that by unbalancing the freform to the cutting tool than would the higher quency gain curve of both the recording apparafrequencies, it is at once apparent that the use tus and the reproducing apparatus in such a of an amplifier, the gain of which varies directly way that the resultant output of the reproducing with the frequency, will produce a record of apparatus is substantially uniform over the whole substantially constant amplitude of track wave frequency range, the difficulties due to non-unithereby permitting a closer spacing of the sound formity of frequency response and record scratch grooves than was heretofore practical.

are substantially overcome. The main object of my invention is to provide 40 It is commonly known that the tonal scale a sound recording and reproducing system capaof the ear is a logarithmetic function of the ble of producingasubstantially uniform response frequency and, therefore, if the audio frequency over the entire frequency range of the reprospectrum consists of frequencies of from 50 cycles duced sound. to 5,000 cycles, 500 cycles will be the central point It is a further object of my invention to proor the half-Way mark of such a tonal scale. Thus, vide a sound recording and reproducing system we see that the lower portion of the audio frethat reduces record surface noise to a minimum. quency spectrum consists of frequencies vary- It is also an object of my inventionrto proing from to 500 cycles or a difference of 450 vide a sound recording system that gives a rec- 50 cycles, While the upper half of the audio fre- 0rd of constant amplitude of track wave as dis- 50 quency spectrum consists of frequencies from 500 tinguished from records of present systems givto 5,000 cycles or a difference of 4,500 cycles. ing constant voltage of pick-up.

It is also known that the surface noise ac- Other objects and advantages of the invention companying a sound record consists of sound relate to the particular circuit arrangement of frequencies fairly evenly distributed throughout my apparatus and will appear more fully in the 55 following description taken in connection with the accompanying drawings in which:-

Figure 1 is a schematic View of my recording system;

Figure 2 is a schematic View of my reproducing system;

Figure 3 shows the gain curve of the audio frequency amplifier used in the recording system;

Figure 4 shows the gain curve of the audio frequency amplifier used in the reproducing system; and

Figure 5 shows the resultant output characteristic of the reproducing system.

Referring to the drawings, the reference character l designates a microphone for picking up the sound to be recorded. An amplifier 2 amplifies the microphone current and feeds the same into a special amplifier comprising a vacuum tube 3 and the necessary coupling units to produce a rising gain characteristic. The tube 3 may be of the well known screen grid type having an amplification factor of approximately 400 and a mutual conductance of about 1,000.

The output circuit of the tube 3 includes resistance 4, inductance 5 and a blocking condenser 6 which is used to prevent the passage of direct current through the inductance 5 from the 250 volt source used to energize the plate of tube 3. Inductance 5 is of such a value that the total output impedance of tube 3 does not vary over 2% from 50 to 5,000 cycles. are taken across inductance 5 and lead to the input of a succeeding tube included in amplifier l2. The input circuit to amplifier !2 comprises a grid condenser 9, a grid resistor l0 and a conventional grid bias battery H. The output of amplifier I2 is connected to a record cutting tool l3 which records the sound waves on a wax record [4 which may be rotated by a turntable l5 in any conventional manner.

In practice, I have used a resistance of 600,000 ohms for resistance 4 and a coil 5 having 1.59 henries inductance. Such an inductance has approximately 50,000 ohms impedance at 5,000 cycles. As shown in Figure 3, this special stage shows a gain of about 20 units at 5,000 cycles and 0.2 unit at 50 cycles or a variation of about 40 decibels. The gain of tube 3 in connection with the above described circuit will be which indicates a rising gain characteristic. In this formula, Mu is equal to the voltage amplification of the tube, 10 is equal to 2 pi times the frequency being amplified, L is the inductance of coil 5, Rp is the internal plate impedance of the tube and r is the value of the external resistance 4.

It will be noted that the input voltage to amplifier I2 is taken across the inductance 5 in the output circuit of tube 3. Thus, the input to the amplifier l2 will be proportional to the voltage drop across this coil. This voltage will vary approximately in accordance with the above formula expressing the gain of the circuit. In the formula, Mu, L, Rp and 7 remain substantially constant over the frequency range from 50 to 5,000 cycles. The term 112, which is 2 pi 1, will vary directly with the frequency 1 being amplified. Thus, the gain will be directly proportional to the frequency from 50 to 5,000 cycles when using the constants above mentioned. A rising gain characteristic with frequency of the special Connections 1 and B r amplifier above described, will eliminate the tendency to overcut the record at the lower frequencies thereby permitting a closer grooving of the record than was heretofore practical.

In order to obtain a fiat frequency response in the reproducing system, I propose to use a circuit arrangement as shown in Figure 2. A permanent disc record as l6 may be made from the wax record It in any manner well known in the art. The record 56 may be rotated by a turn table ll of suitable design. A pick-up device [8 may be conveniently connected to amplifier tube 20 by means of transformer IS. The tube 20 is of the screen grid type and, in this instance, may have characteristics similar to that of the tube 3 in-the recording apparatus. The value of resistance 2| that I have chosen may be 600,000 ohms and the capacity of condenser 22 is such that the output circuit of the tube 20 maintains practically constant impedance throughout the audio band. This capacity has been found in practice to be equal to .064 mid. and has an impedance of approximately 50,000 ohms at 50 cycles.

The input to the succeeding amplifier tube of amplifier 28 is taken by means of leads 23 and 24 across the condenser 22. In addition, this input circuit also includes a conventional grid coupling condenser 25, a grid resistor 26 and a grid bias battery 27. The output of amplifier 28 may be connected in the usual manner to a reproducing device or loud speaker 29.

From the circuit diagram shown in Figure 2, it is apparent that the gain of the special am plifier stage in the recording apparatus is represented by the formula where Mu is equal to the voltage amplification of the tube 20, Rp is the internal plate impedance of the tube, r is the value of the external resistance 2 l, w is equal to 2 pi times the frequency being amplified and c is the value of the capacity 22.

It will be noted that the input voltage to amplifier 28 is taken across the condenser 22 in the output circuit of the tube 20. The voltage drop across the condenser 22 is proportional to the output of the tube 20 as given in the approximate formula above. In the formula, Mu, Rp, r, and 0 remain approximately constant throughout the frequency range from 50 to 5,000 cycles. The term 10 which is 2 pi f, will vary directly with the frequency f being amplified. Now then, since the gain of the tube 20 varies inversely with the frequency, then as the frequency increases, the voltage drop across the condenser 22 will decrease.

Using the constants given in the previous discussion and calculating the gain by means of the formula, we find that the gain varies from 20 units at 50 cycles to 0.2 unit at 5,000 cycles or a loss of 40 decibels at 5,000 cycles overthe 50 cycle level. The gain characteristic of this amplifier is shown in Figure 4 and it will be noted that this characteristic is just the reverse of the gain characteristic of the recording amplifier. It is, therefore, apparent that the resultant response at the loud speaker will be of the form shown by the curve in Figure 5. This is what is commonly called a fiat frequency response and is the ultimate characteristic sought by sound engineers.

As pointed out above, in the conventional method of cutting records, the amplitude of the track wave is approximately inversely proportional to the frequency of the recorded sound, and the track wave amplitude for the lower frequencies is greater than that for the higher frequencies. When reproduced electrically, these records therefore produce a substantially constant voltage of pick-up. In recording with the present invention, on the other hand, the amplification is directly proportional to the frequency of the sound to be recorded, so that the higher frequencies are amplified proportionately more than the lower frequencies, resulting in a substantially constant amplitude of the track wave. This eliminates the tendency present in the conventional system to overcut the record at the lower frequencies, and the present invention therefore permits a closer grooving of the record than was heretofore practical. Most of the objectionable record scratch is present at the higher frequencies, and the rising gain with frequency characteristic of the amplification of the present invention during recording results in raising the energy level of the higher frequencies of the recorded sound relative to the energy level of the record scratch. When the record so made is reproduced according to the present invention, therefore, using an amplifier having a gain characteristic inversely proportional to the frequency of the recorded sound, not only is a uniform or fiat frequency response over the entire frequency range of the recorded sound obtained for the system as a whole, but the undesirable record scratch is reduced to a minimum without impairing the quality of the reproduced sound because the record scratch occurs largely at high frequencies which are amplified after pick-up only relatively little and because it is present in the record at a low energy level as compared to the desired recorded sounds of similar frequency.

It is to be understood that the specific apparatus herein disclosed is merely for the purpose of tool, and means for actuating said cutting tool in accordance with the voltage variations across said inductance, the constants of said amplifier output circuit being such that the amplifier has a gain characteristic rising in direct proportion to the frequency of said electrical waves for producing voltage variations across said inductance such that the record track wave cut by said cutting tool has a substantially constant amplitude throughout the recorded frequency band.

2. In a sound reproducing system for the reproduction of permanent sound records having a substantially constant amplitude of track wave, means for producing electrical waves corresponding in frequency with the track waves on the record, an amplifier for amplifying said electrical waves, said amplifier having at least one vacuum tube and having an output circuit including a resistance and a condenser in series, and a translating device for producing sound actuated by the voltage variations across said condenser, the con stants of said amplifier output circuit being such that the amplifier has a gain characteristic rising in inverse proportion with the frequency of the recorded waves for producing voltage variations across said condenser such that'a substantially fiat frequency response is obtained at the translating device.

STUART W. SEELEY. 

